Advanced anticorrosion coatings on lightweight magnesium alloys by atmospheric co2 plasma treatment

ABSTRACT

An improved method for preventing corrosion of magnesium is provided. The method includes providing a magnesium substrate including a native surface layer of nanoporous MgO and Mg(OH) 2 . The method includes generating a CO 2  plasma at atmospheric pressure, flowing the CO 2  plasma from a nozzle exit as a plasma plume, and exposing the surface film to the plasma plume. The method further includes reacting activated CO 2  gas molecules with the native surface layer by performing an atmospheric CO 2  plasma treatment at room temperature to convert at least a portion of the native surface layer of nanoporous MgO and Mg(OH) 2  into a nano-structured to micro-structured MgO/MgCO 3  coating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application 63/062,455, filed Aug. 7, 2020, the disclosure of which is incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to the plasma treatment of a magnesium substrate for the prevention of corrosion.

BACKGROUND OF THE INVENTION

Magnesium is among the lightest and the lowest cost structural materials with excellent mechanical properties for various fields, including ground/air/marine transportation, electronic components, medical implants, hydrogen storage, and battery electrodes. Particularly in automotive applications, the replacement of structural steels with magnesium alloys enables a 60% weight reduction for existing vehicle components, which can improve fuel efficiency and decrease gas emissions. While approximately 350 lbs. of magnesium alloy parts have been approved for use in chassis, interiors, bodies and powertrain components, there are still only an average of 11-14 lbs. of magnesium components used in each North American vehicle. One of the main issues is the poor corrosion resistance of magnesium alloys. Generally, magnesium alloys have poor corrosion resistance due to their high chemical reactivity and the lack of a protective passive oxide film. In air, the surface of magnesium naturally forms Mg(OH)₂ and MgO layers, which are porous and unprotective, especially in humid environments.

SUMMARY OF THE INVENTION

An improved method for preventing corrosion of magnesium is provided. The method includes providing a magnesium substrate including a native surface layer of nanoporous MgO and Mg(OH)₂. The method further includes reacting activated CO₂ gas molecules with the native surface layer by performing an atmospheric CO₂ plasma treatment (CO₂-AP) at room temperature to convert at least a portion of the native surface layer of nanoporous MgO and Mg(OH)₂ into a nano- to micro-structured MgO/MgCO₃ coating. In some embodiments, the method includes reacting activated CO₂ gas molecules with the native surface layer in successive sweeps of a plasma plume. The resulting nano- to micro-structured MgO/MgCO₃ coating can include a thickness of between 0.1 μm and 10 μm, inclusive, further optionally between 0.5 μm and 5 μm, inclusive, comprising MgCO₃ and MgO uniformly dispersed therein. In other embodiments, the magnesium substrate is prepared by forming a surface film of nanoporous MgO and Mg(OH)₂. The step of preparing the magnesium substrate optionally includes forming the surface film via salt water immersion (e.g., aqueous NaHCO₃) followed by distilled water immersion of the magnesium substrate.

These and other features of the invention will be more fully understood and appreciated by reference to the description of the embodiments and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a method for preventing corrosion of a magnesium substrate using a CO₂ plasma treatment.

FIG. 2 is a graph depicting the results of hydrogen collection tests performed on magnesium substrates subjected to a variety of CO₂ plasma treatment methods.

FIG. 3 includes STEM images a) through h) depicting a micro-structured MgO/MgCO₃ layer on magnesium substrates generated by a CO₂ plasma treatment.

FIG. 4 is a material characterization depicting the formation of a micro-structured MgO/MgCO₃ layer on a magnesium substrate generated by a CO₂ plasma treatment.

DETAILED DESCRIPTION OF THE CURRENT EMBODIMENT

As discussed herein, the current embodiment generally relates to a method for preventing corrosion of a magnesium substrate using a CO₂ plasma treatment. The method includes providing or preparing a magnesium substrate including a native surface layer or surface film of nanoporous MgO and Mg(OH)₂ and reacting activated CO₂ gas molecules with the native surface layer or surface film by performing an atmospheric CO₂ plasma treatment at room temperature to convert at least a portion of the native surface layer or surface film of nanoporous MgO and Mg(OH)₂ into a nano- to micro-structured MgO/MgCO₃ coating.

The method for preventing corrosion of magnesium generally includes providing a magnesium substrate including a native surface layer of nanoporous MgO and Mg(OH)₂. The magnesium substrate includes magnesium or a magnesium alloy. The magnesium alloy can further include aluminum, zinc, manganese, silicon, copper, rare earths, carbon, iron, or zirconium. As used herein, a “native” surface layer is a section of the magnesium substrate that naturally forms from the reaction of water in ambient air with the magnesium or the magnesium within the magnesium substrate, i.e., a surface layer solely resulting from the exposure of the magnesium substrate to ambient air at standard temperature and pressure.

In other embodiments, the method includes actively forming a surface film of nanoporous MgO and Mg(OH)₂. The magnesium substrate is contacted with water, either in a vapor or a liquid state, and the magnesium reacts with the water to form the surface film of nanoporous MgO and Mg(OH)₂. In such embodiments, the water is distilled water and the surface film is formed via the immersion of the magnesium substrate into deionized water. In other embodiments, the magnesium substrate is first contacted with salt water (e.g., aqueous NaHCO₃) to shorten the pretreatment immersion in distilled water from 24 hours to less than 10 minutes.

The method for preventing corrosion of magnesium includes the step of reacting activated CO₂ gas molecules with the native surface layer or the surface film of the magnesium substrate by performing an atmospheric CO₂ plasma treatment at room temperature to convert at least a portion of the native surface layer of nanoporous MgO and Mg(OH)₂ into a nano- to micro-structured MgO/MgCO₃ coating. The atmospheric CO₂ plasma treatment is open to the air. Generally, the atmospheric CO₂ plasma treatment is carried out in a CO₂ enriched cover gas chamber to maximize MgCO₃ formation. In some embodiments, the atmospheric CO₂ plasma treatment of the native surface layer or surface film is performed manually. In alternative embodiments, the atmospheric CO₂ plasma treatment of the native surface layer or surface film is performed by an automated system. In such embodiments, the automated system is conducted as a batch process. In certain embodiments, the automated system is conducted as a continuous process. Generally, the treatment time is 10 to 60 minutes, alternatively 15 to 45 minutes, alternatively around 30 minutes.

The method also includes generating a CO₂ plasma at atmospheric pressure. A CO₂ gas feedstock is provided in a cover gas chamber at atmospheric pressure and at room temperature, and the CO₂ plasma is generated by applying an electrical field to the CO₂ gas feedstock. The electrical field is generated by at least two electrodes that are spaced apart from each other. The electrodes produce a discharge voltage of from 1 to 10,000 V, alternatively 200 to 1000 V. The method also includes flowing the CO₂ plasma from a nozzle exit as a plasma plume. Generally, the plasma plume is substantially oriented toward the magnesium substrate. The method optionally includes exposing the native surface layer or the surface film to the plasma plume. In certain embodiments, the step of reacting activated CO₂ gas molecules with the native surface layer or surface film is performed in successive sweeps of a plasma plume. In some embodiments, less than 30 sweeps are made, alternatively less than 10, alternatively less than 5.

The nano- to micro-structured MgO/MgCO₃ coating can include a thickness of between 0.1 μm and 10 μm, inclusive, alternatively between 0.5 μm and 5 μm, inclusive. The MgO/MgCO₃ coating includes MgCO₃ and MgO uniformly dispersed therein. In other embodiments, the MgO/MgCO₃ coating is distinct from an MgO coating. In these embodiments, the MgO coating is between the MgO/MgCO₃ coating and a magnesium body.

Magnesium substrates treated with the atmospheric CO₂ plasma treatment were discovered to possess an excellent corrosion rate mitigation effect. In laboratory testing, magnesium substrates treated with an atmospheric CO₂ plasma treatment demonstrated superhydrophobicity and possessed minimal water dispersion when compared to untreated magnesium substrates. The treated magnesium substrates also exhibited a thin layer of air on a surface of the magnesium substrate when immersed in an aqueous salt solution. Additionally, the atmospheric CO₂ plasma treatment was discovered to facilitate the chemical and/or physical adsorption of acidic CO₂ molecules on a surface of the magnesium substrate and the formation of various carbonate surface species (e.g. potentially including bicarbonate, bidentate, and/or unidentate carbonates).

The present invention is further described below in connection with a laboratory example. The laboratory example simulated a highly aggressive environment to evaluate corrosion resistance for long-term exposure. As a comparative sample, a magnesium substrate was first immersed in a 3.5 wt % NaCl saltwater solution for 25 hours. A hydrogen collection test was then performed on the treated magnesium substrate, with the results shown in FIG. 2 (“Comparative Example”). Three treated samples were then evaluated against the comparative sample.

The first treated sample included a magnesium substrate that was immersed in distilled water to form a surface film of nanoporous MgO and Mg(OH)₂. A CO₂ plasma plume was generated at atmospheric pressure, and the surface film was exposed to a single pass of the plasma plume. The plasma plume (generated by 100% CO₂ gas) passed through the sample surface with 0.38 cm height, 762 cm/min, and 0.06 cm spacing at 70 SLPM (standard liters per minute) of flow rate. The magnesium substrate was then immersed in a 3.5 wt % NaCl saltwater solution for 25 hours. A hydrogen collection corrosion test was then performed on the treated magnesium substrate. The results are shown in FIG. 2 (“Example 1”). The second treated sample included a magnesium substrate that was immersed in distilled water to form a surface film of nanoporous MgO and Mg(OH)₂. A CO₂ plasma plume was generated at atmospheric pressure, and the surface film was exposed to five passes of the plasma plume. The magnesium substrate was then immersed in a 3.5 wt % NaCl saltwater solution for 25 hours. A hydrogen collection corrosion test was then performed on the treated magnesium substrate. The results are shown in FIG. 2 (“Example 2”). A third treated sample included a magnesium substrate that was exposed to the open air such that a native surface layer was formed. A CO₂ plasma plume was generated at atmospheric pressure, and the native surface layer was exposed to a single pass of the plasma plume. The magnesium substrate was then immersed in a 3.5 wt % NaCl saltwater solution for 25 hours. A hydrogen collection corrosion as was then performed on the treated magnesium substrate. The results are shown in FIG. 2 (“Example 3”).

As shown in FIG. 3, scanning transmission electron microscope (STEM) images illustrate the MgO/MgCO₃ coating on magnesium substrates generated by CO₂ plasma treatment. This figure shows that chemically converted nanoparticle agglomerations are formed pillars on a dense layer that is formed on the Mg surface. The elemental mapping of the structured layer with ˜1-2 μm thickness indicates that three major elements (Mg, 0, and C) are well-dispersed throughout the layer, verifying the coexistence of MgO and MgCO₃. In particular, high-magnification cross-sectional HAADF-STEM images (a, e) of the treated Mg (five-times swing) along with corresponding energy-dispersive X-ray spectroscopy elemental mapping (b-d, f-h) show the distribution of Mg, C, and O in the treated layer (C: 1.5 atomic %, O: 53.0 atomic %, and Mg: 26.5 atomic % in the zoomed-in image). As also shown in FIG. 4, X-ray diffraction (XRD) of the MgO/MgCO₃ coating confirmed that the phase content of the resultant layer was primarily MgO and MgCO₃. XRD patterns of untreated Mg were measured at two angles (0° and 90°) to avoid the crystalline directional effect.

The results of the foregoing laboratory example demonstrated a greater than ten-fold increase in corrosion resistance of treated magnesium when compared to untreated magnesium. The excellent corrosion rate mitigation effect is believed to be associated with the non-wettable surface properties (super hydrophobicity), chemical composition, and crystalline changes from pure magnesium due to the atmospheric CO₂ plasma treatment.

The above description is that of current embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. This disclosure is presented for illustrative purposes and should not be interpreted as an exhaustive description of all embodiments of the invention or to limit the scope of the claims to the specific elements illustrated or described in connection with these embodiments. For example, and without limitation, any individual element(s) of the described invention may be replaced by alternative elements that provide substantially similar functionality or otherwise provide adequate operation. This includes, for example, presently known alternative elements, such as those that might be currently known to one skilled in the art, and alternative elements that may be developed in the future, such as those that one skilled in the art might, upon development, recognize as an alternative. Further, the disclosed embodiments include a plurality of features that are described in concert and that might cooperatively provide a collection of benefits. The present invention is not limited to only those embodiments that include all of these features or that provide all of the stated benefits, except to the extent otherwise expressly set forth in the issued claims. Any reference to claim elements in the singular, for example, using the articles “a,” “an,” “the” or “said,” is not to be construed as limiting the element to the singular. 

1. A method for preventing corrosion of magnesium, the method comprising: providing a magnesium substrate including a native surface layer of nanoporous MgO and Mg(OH)₂; and reacting activated CO₂ gas molecules with the native surface layer by performing an atmospheric CO₂ plasma treatment at room temperature to convert at least a portion of the native surface layer of nanoporous MgO and Mg(OH)₂ into a nano- to micro-structured MgO/MgCO₃ coating.
 2. The method of claim 1, wherein reacting activated CO₂ gas molecules with the native surface layer is performed in successive sweeps of a plasma plume.
 3. The method of claim 1, wherein the atmospheric CO₂ plasma treatment includes generating a CO₂ plasma at atmospheric pressure, flowing the CO₂ plasma from a nozzle exit as a plasma plume, and exposing the native surface layer to the plasma plume.
 4. The method of claim 3, wherein generating the CO₂ plasma includes applying an electrical field to a CO₂ gas feedstock.
 5. The method of claim 1, wherein the nano- to micro-structured MgO/MgCO₃ coating comprises a thickness of between 0.1 μm and 10 μm, inclusive.
 6. The method of claim 1, wherein the MgO/MgCO₃ coating comprises MgCO₃ with MgO uniformly dispersed therein.
 7. The method of claim 1, wherein the magnesium substrate comprises a magnesium alloy.
 8. A method for preventing corrosion of magnesium, the method comprising: preparing a magnesium substrate by forming a surface film of nanoporous MgO and Mg(OH)₂; and reacting activated CO₂ gas molecules with the surface film by performing an atmospheric CO₂ plasma treatment at room temperature to convert the surface film of nanoporous MgO and Mg(OH)₂ into a nano-structured to micro-structured MgO/MgCO₃ coating.
 9. The method of claim 8, wherein forming the surface film includes distilled water immersion of the magnesium substrate.
 10. The method of claim 8, wherein the nano- to micro-structured MgO/MgCO₃ coating comprises a thickness of between 0.1 μm and 10 μm, inclusive.
 11. The method of claim 8, wherein reacting activated CO₂ gas molecules with the surface film is performed in successive sweeps of a plasma plume.
 12. The method of claim 8, wherein the atmospheric CO₂ plasma treatment includes generating a CO₂ plasma at atmospheric pressure, flowing the CO₂ plasma from a nozzle exit as a plasma plume, and exposing the surface film to the plasma plume.
 13. The method of claim 13, wherein generating the CO₂ plasma includes applying an electrical field to a CO₂ gas feedstock.
 14. The method of claim 8, wherein the nano-structured to micro-structured MgO/MgCO₃ coating comprises MgCO₃ with MgO uniformly dispersed therein.
 15. The method of claim 8, wherein the magnesium substrate comprises a magnesium alloy. 